This invention relates generally to solid state radiation imagers and, in particular, to a cover plate having a spacer lip with a hermetic barrier (e.g., a moisture barrier) and which encloses the imager array and protects it from adverse environmental conditions.
Solid state radiation imaging arrays typically comprise a photosensor array coupled to a scintillator. The radiation to be detected (e.g., x-rays or the like) penetrates the scintillator and is absorbed by the scintillator material in an event that results in the release of optical photons. The photosensor array coupled to the scintillator is used to detect the optical photons, providing a spatial location (x,y) in the array that corresponds with the point in the scintillator at which the incident radiation was absorbed. Readout of the photosensor array allows electrical signals to be generated that correspond to the pattern of absorbed radiation. The data embodied in such electrical signals can be presented in a visual display or otherwise processed to allow analysis of the radiation pattern.
The imager includes a glass substrate on which the photosensor array, typically comprising layers of thin films (including amorphous silicon) patterned into thin film transistors (TFT) transistors and photodiodes, is disposed. The scintillator layer typically comprises a salt, such as cesium iodide (CsI), that is deposited over the photosensor array on the substrate. The scintillator converts x-rays into visible light that is in turn detected by the photodiodes. The scintillator structure can be rapidly degraded by moisture, resulting in degraded optical performance and consequent degraded imager performance. Degradation can occur even during exposure to ambient room humidity. Accordingly, it is important that the scintillator material be protected from exposure to ambient conditions by a hermetic cover material.
The cover material for the imager array is, desirably, highly transmissive to x-rays. It further must be structurally robust such that it retains its form and effectiveness to provide the desired moisture resistance in a variety of environments, such as conditions (e.g., temperature extremes) that can be experienced during shipping and conditions of prolonged use and radiation exposure. For example, the cover material must be able to retain its form and moisture resistant qualities through thermal cycling without deformation (such as delamination or deformation) that results in a breach of the hermetic barrier around the scintillator. Any breach of the seal around the scintillator can result in degradation of the scintillator material and adversely affect performance of the imager. The cover material must adhere well to glues (e.g., epoxy material) used to cement the device together and must be able to retain these desirable characteristics over the expected life and operational environment (e.g., radiation exposure) of the imager. Additionally, the cover material should have the characteristics of being readily formed in large, thin sheets required for large area imagers (e.g., about 100 cm2 or greater), such as typically are used for chest x-ray systems.
A difficulty is experienced with some imager arrangements when assembling the cover plate with the substrate using a single bead of epoxy to seal the two pieces as the compression of the air between the two pieces can cause displacement of the epoxy bead.
Other approaches to improving the seal typically include additional fabrication steps, resulting in additional handling and time requirements, contributing to increased costs.
Accordingly, there is a need to provide a satisfactory hermetic seal between the periphery of the cover plate and the mating surface of the substrate in a solid state radiation imager assembly which provides an affective seal and also is efficient and easy to manufacture.
In one embodiment of the invention, a cover plate for a radiation imager having a photosensor array disposed on a substrate and a scintillator optically coupled to the photosensor array comprises a lip on the inner mains surface of the cover plate, with a at least a portion of the lip comprising a hermetic barrier; and a moisture resistant adhesive layer disposed to seal the lip to the substrate.
A process for forming a cover plate for a solid state radiation imager includes the steps of providing a cover plate; forming a lip on the inner main surface of the cover plate adjacent a periphery of the cover plate, with at least a portion of the lip comprising a hermetic barrier; applying a layer of a moisture resistant adhesive to the hermetic barrier and a portion of the substrate of the array; and assembling the cover plate with the surface of the substrate.